The present invention relates to apparatus for determining the proportion of a given gas component in a gas mixture.
It is well known that when wishing to detect the presence of and/or determine the proportion of a given component of agas mixture, there can be used an apparatus or device which incorporates a piezoelectric crystal, e.g. a quartz crystal, having provided on its surfaces a thin substrate layer comprising one or more substances capable of reversibly and selectively adsorbing the gas component of interest, i.e. the relevant gas component, from a gas mixture, the crystal being so arranged as to enable the gas mixture being assayed to be brought into contact with the substrate layer. The substrate layer therewith adsorbs molecules of the relevant gas component from the gas mixture to an extent contingent on the concentration of said component in the mixture and increases in mass in dependence hereon, which results in a decrease in the vibration frequency of the piezoelectric crystal. The vibration frequency of the crystal is normally determined by incorporating the crystal in an oscillator circuit constructed to produce an output signal representative of the oscillating frequency of the oscillator circuit, and therewith also of the crystal, in relation to a reference frequency. The reference frequency is normally the oscillating frequency to another, similar crystal, which lacks the aforedescribed substrate layer but which, in all other respects, is arranged identically with the crystal provided with said substrate layer. An output signal representing the frequency difference of the two crystals will therefore constitute a measurement of the quantity of the relevant component present in the assayed gas mixture. Apparatus of this kind are described for example, in U.S. Pat. Nos. 3 164 004 and 4 399 686. Substances suitable for use as a substrate layer for the selective and reversible adsorption of many different gaseous components of a gas mixture are also known to the art.
One problem with gas analyzing apparatus of this kind is that the ability of the substrate material to adsorb the relevant gas component from a gas mixture is highly dependent on the temperature of the substrate material, such that with an unchanged concentration of the relevant gas component in the gas mixture the quantity of the component adsorbed decreases with increasing substrate layer temperatures. The sensitivity of the gas analyzer is thus correspondingly temperature dependent, and decreases with increasing substrate temperatures. In conjunction with gas analyzers for assaying respiration gas mixtures in order to determine the constituent proportion of anaesthetic gases of the type halogenated hydrocarbon compounds, such as halothane, enflurane, methoxy flurane and iso-flurane, said substrate layer being composed, for example, of different types of silicon oils, it has been found that the aforesaid temperature dependency has a form which can be expressed by the equation EQU S.about.T.sup.-5/2 .multidot.e.sup.1/T ( 1)
where S is the above-mentioned output signal representing the difference frequency of the crystals, and T is the temperature (.degree.K.) of the substrate layer. Within a temperature range of 20.degree.-45.degree. C., which is an appropriate temperature range with gas analyzers of this kind, the change in analyzer sensitivity as a result of variations in temperature is in the order of 150%.
It will be realized that the aforesaid temperature dependency must be eliminated in some way. One possibility is to maintain the piezoelectric crystal and its substrate layer at a constant temperature. This can be effected by either cooling or heating the crystal. Cooling of the crystal results in the condensation of water vapor on the crystal, which creates problems, at least when analysing respiration gas mixtures, and renders an accurate assay totally impossible. When heating the crystal it is necessary to use temperatures of up to 50.degree. C., at least when applicable to the aforesaid analysis of respiration or breathing gas mixtures, which results in a marked reduction in the sensitivity of the analyzer and also in far more rapid deterioration of the properties of the substrate layer, therewith reducing the useful life-span of the crystal. Both of these methods for maintaining the crystal and substrate layer at a constant temperature also have a considerable power consumption which, among other things, renders the use of a battery-driven gas analyzer impossible. A further difficulty in conjunction with maintaining the crystal and the substrate layer at a constant temperature resides in the fact that the crystal has a very low thermal capacity, and hence its temperature accompanies variations in ambient temperature extremely rapidly, i.e. in practice variations in the temperature of the analysed gas mixture.
Consequently, the object of the present invention is to provide an apparatus of the kind mentioned in the introduction with which the problem associated with temperature-dependent sensitivity is eliminated or, in all events, greatly reduced, without needing to maintain the crystal and its substrate layer at a constant temperature.